1. Field of the Invention
The present invention relates to a method for forming a film made of a thermoplastic polymer capable of forming an optically anisotropic melt phase (which thermoplastic polymer is hereinafter referred to as a thermotropic liquid crystal polymer) or a polymer composition containing such thermotropic liquid crystal polymer.
2. Description of the Related Art
In recent years, in the electric and electronic fields, demands for reduction in size and/or weight of electric and electronic instruments and equipments are increasing and, in the course of this, the need has arisen to make available an insulating film having a uniform film thickness and excellent electrical and mechanical properties. Although with polyimide and polyethylene terephthalate which are generally used as a material for the conventional insulating film can be effectively utilized to provide a film having a uniform film thickness, such material involves a problem in that not only is the electrical property insufficient in a high frequency region, but also the electrical property tends to be deteriorated and change in dimension tends to occur because of a particularly high moisture absorbability. Accordingly, with such material, the film satisfying the above discussed demands is considered unable to manufacture.
In contrast thereto, in view of the fact that the thermotropic liquid crystal polymer has an excellent electrical property and a low dimensional change ratio and also has high heat resistance and chemical stability, the thermotropic liquid crystal polymer is considered useful in the electric and electronic fields. As a method of simply manufacturing a film made of the thermotropic liquid crystal polymer, the film forming method utilizing the inflation molding technique is suggested in the Japanese Laid-open Patent Publication No. 2-3430.
However, with the conventional film forming method, when the thermotropic liquid crystal polymer in a molten state is to be extruded from an inflation molding machine, the melt viscosity thereof tends to abruptly decrease by the effect of a shear stress developed in the die, rendering the film to be difficult to retain its shape. Also, the melt viscosity of the extruded thermotropic liquid crystal polymer abruptly increased during cooling that takes place subsequent to extrusion through the die and, therefore, the anisotropy of physical properties of the film in longitudinal and lateral directions, which is brought about by the molecular orientation, is difficult to improve. Moreover, it is difficult to suppress variation in thickness of the film and variation in anisotropy in physical property of the film, both resulting from a slight difference in melt viscosity. Those difficulties make it difficult to simply and stably manufacture the film having a minimized anisotropy, that is, having a property as isotropic as possible, with uniform film thickness and anisotropy.